Dosing A+B style liquids will add sodium and cloride to your tank over time, these are conservative elements and will build up... This can theoretically become a problem. Calcium reactors are very cost effective to run (although not cheap) and add nothing non-conservative to the tank. They depress your pH slightly, although if run properly not a biggie. Kalk reactors are great for low-medium demand tanks. In a nutshell, I'd go for the kalk reactor as long as you are mindful of the danger of overdosing. Calcium reactors coupled with kalk reactors have a synergistic effect, so it can be nice to have a kalk reactor prior to a calcium reactor.
